Technical Abstract:
Leucine (Leu) acutely stimulates protein synthesis by activating the mammalian target of rapamycin complex 1 (mTORC1) pathway. To determine whether Leu can stimulate protein synthesis in muscles of different fiber types and visceral tissues of the neonate for a prolonged period and to determine the role of other amino acids in the response, overnight fasted neonatal pigs were infused for 24 h with saline, Leu (400 umol/kg(-1)/h(-1)), or Leu with replacement amino acids to prevent the Leu induced fall in other amino acids. Protein synthesis rates and phosphorylation of 4E binding protein (4E-BP1) and S6 kinase 1 (S6K1), indicators of mTORC1 activation, were measured. Leu increased 4E-BP1 and S6K1 phosphorylation in the longissimus dorsi, gastrocnemius, and masseter muscles, liver, and pancreas, in both the absence and presence of amino acid replacement. However, protein synthesis was increased only when amino acids were infused to prevent hypoaminoacidemia. Leu had no affect on mTORC1 signalling or protein synthesis in the heart, jejunum, and kidney. Thus, prolonged infusion of leucine stimulates mTORC1 signaling in skeletal muscle and some visceral tissues but the Leu-induced stimulation of protein synthesis in these tissues requires sustained amino acid availability.